My friend is due to have a baby girl in a few months, and she is really into fairies. I'd love to knit her a ballet style wrap over cardie, and then make some really light, whispy, fairy wings for the back.

I think you're on the right track with the mohair... anything laceweight would probably be light enough, but something with a little fuzz, like mohair, Rowan's KidSilkHaze, Madil Kid Seta, or Douceur et Soie would probably look really nice.